1. Field of the Invention
The invention relates generally to downhole tools. More particularly, the present invention relates to progressive cavity pumps. Still more particularly, the present invention relates to tag systems for positioning and locating the rotor relative to the stator of a progressive cavity pump.
2. Background of the Invention
A progressive cavity pump (PC pump), also know as a “Moineau” pump, transfers fluid by means of a sequence of discrete cavities that move through the pump as a rotor is turned within a stator. Transfer of fluid in this manner results in a volumetric flow rate proportional to the rotational speed of the rotor within the stator, as well as relatively low levels of shearing applied to the fluid. Consequently, progressive cavity pumps are typically used in fluid metering and pumping of viscous or shear sensitive fluids, particularly in downhole operations for the ultimate recovery of oil and gas. A PC pump may be used in reverse as a positive displacement motor (PD motor) to convert the hydraulic energy of a high pressure fluid into mechanical energy in the form of speed and torque output, which may be harnessed for a variety of applications, including downhole drilling.
As shown in FIGS. 1 and 2, a conventional PC pump 10 comprises a helical-shaped rotor 30, typically made of steel that may be chrome-plated or coated for wear and corrosion resistance, disposed within a stator 20, typically a heat-treated steel tube or housing 25 lined with a helical-shaped elastomeric insert 21. The helical-shaped rotor 30 defines a set of rotor lobes 37 that intermesh with a set of stator lobes 27 defined by the helical-shaped insert 21. As best shown in FIG. 2, the rotor 30 typically has one fewer lobe 37 than the stator 20. When the rotor 30 and the stator 20 are assembled, a series of cavities 40 are formed between the outer surface 33 of the rotor 30 and the inner surface 23 of the stator 20. Each cavity 40 is sealed from adjacent cavities 40 by seals formed along the contact lines between the rotor 30 and the stator 20. The central axis 38 of the rotor 30 is parallel to and radially offset from the central axis 28 of the stator 20 by a fixed value known as the “eccentricity” of the PC pump.
During operation of the PC pump 10, the application of torque to rotor 30 causes rotor 30 to rotate within stator 20, resulting in fluid flow through the length of PC pump 10. In particular, adjacent cavities 40 are opened and filled with fluid as rotor 30 rotates relative to stator 20. As this rotation and filling process repeats in a continuous manner, fluid flows progressively down the length of PC pump 10.
PC pumps are used extensively in the oil and gas industry for operating low pressure oil wells and also for raising water from wells. As shown in FIG. 3, PC pump 10 previously described disposed in a cased borehole 50 in a conventional manner to pump oil to the surface. Since PC pumps (e.g., PC pump 10) are often mounted tens or hundreds of meters below the surface, it is difficult to mount an electric drive motor to the PC pump. Consequently, as shown in FIG. 3, it has become common practice to secure the stator 20 on to the lower end of a string of production tubing 60. In particular, the upper threaded end of the stator housing 25 is axially connected end-to-end with the lower threaded end of the production tubing 60 with a mating threaded collar 65. Once the stator 20 is secured to the lower end of the production tubing 60, it is lowered into the cased borehole 50 on the tubing string 60. Thus, the production tubing 60 is used both to position stator 20 and PC pump 10 at a specific depth in the well bore, and to axially support the weight of the PC pump 10 and the weight of the fluid column extending between the PC pump 10 and the surface which bears against the upper end of stator liner 21.
Once the stator 20 is properly positioned at the desired depth for production, the upper end of the rotor 30 is threaded to the lower end of a sucker rod string 70 at the surface, lowered through the production tubing 60, and inserted into the stator liner 21. To operate PC pump 10 at the desired capacity, rotor 30 must be positioned at the proper axial position relative to stator 20. For example, if the lower end of rotor 30 does not extend to the lower end of stator liner 21, a portion of the lower end of the liner 21 will not be in engagement with rotor 30, and thus, pumping capacity may suffer. Thus, to properly position the rotor 30 within the stator 20, a tag-bar 80 is provided at the lower end of the stator 20. The tag-bar 80 extends across the lower portion of the stator 20, and thus, the rotor 30 is axially lower until the lower end of rotor 30 contacts the tag-bar 80. Once the lower end of the rotor 30 contacts tag-bar 80 and the weight of sucker rod string 70 has been supported by the tag-bar 80 as detected at the surface, the entire rod string 70 is lifted upward a predetermined distance to account for stretching of sucker rod string 70 and to properly position the entire rotor 30 within the stator 20. To begin pumping, a drivehead at the surface applies rotational torque to the rod string 70, which in turn causes downhole rotor 30 to rotate relative to the stator 20.
One disadvantage of the conventional approach employing the tag-bar 80 extending across the lower end of the stator 20 to position the rotor 30 within the stator 20 is that the tag-bar 80 creates an obstruction in the stator 20 and the production tubing 60. Consequently, tag-bar 80 prevents the lowering of tools and/or instruments axially below the stator 20.
Accordingly, there remains a need in the art for improved systems, devices, and methods for the downhole positioning PC pump rotors within PC pump stators. Such devices, methods, and systems would be particularly well received if capable of allowing the insertion of tools and instruments through the stator and into the portion of the wellbore below the stator.